Pultrusion, first developed in the early 1950's, is now a well-known process for the fabrication of reinforced plastic articles. Pultrusion methods and apparatus involve continuously pulling reinforcement materials in the form of continuous webs or strands from a supply station through a bath of catalyzed resin so as to impregnate the reinforcement material with the resin, and pulling the impregnated materials through a heated forming die in which the resin is polymerized to form a cured article. An appropriate puller mechanism is a component of pultrusion apparatus and engages the cured article after it leaves the die so as to pull the composite through the machine, including pulling the reinforcement materials from the supply station through the resin bath and the die. Pultrusion is an effective system for the continous production of solid or hollow articles that have a constant cross-sectional shape such as I-beams, sheets, plates, rods, channels, round and square tubes, and the like. The continuous cured reinforced plastic article is cut into selected lengths by a cut-off device, such as a saw, located downstream of the puller mechanism.
Unsaturated polyester resins are the resins most widely used in pultrusion processes, although vinyl ester resins, epoxy resins and some thermoplastic resins are also used but to a much lesser extent. Numerous types of unsaturated polyester resins suitable for pultrusion are commercially available. The specific polyester used for a particular product should be selected by reference to the expected service conditions, including consideration of factors such as temperature resistance, weather resistance, abrasion resistance, etc., required for the end product. Polyester resins are mixed with a styrene monomer which acts as a cross-linking agent for curing the polyester into a hard rigid mass. Thus the resin bath of a pultrusion machine will typically include an unsaturated polyester, styrene monomer and catalyst, together with fillers, additives, mold release agents and pigments as required.
As mentioned above, the reinforcing materials used in a composite made with pultrusion apparatus are continuously pulled through the heated die of the machine by the puller mechanism which is located downstream of the die so as to engage the cured article emerging from the die. This requires that the reinforcing material employed in a pultruded plastic composite article be supplied in the form of continous webs or strands in order to be pulled by the puller mechanism of the machine from a supply station through the heated die. The reinforcing materials typically employed are unidirectional, bidirectional and multidirectional materials. Continuous strand fiberglass rovings are an example of unidirectional reinforcement materials. Rovings are made from strands comprising a number (e.g. 50 or more) of continuous filaments or bundles of continuous filaments that are gathered together without mechanical twist and wound onto a cylindrical package or spool. Most pultruded fiber reinforced plastic products will include a substantial percentage of rovings, which provide for axial reinforcement and tensile strength of the finished product. Multidirectional reinforcement materials comprise continuous strand mats and may be employed in a pultruded part in order to improve transverse or cross directional physical properties. Resinous binders are generally used to hold the strands together and form a continous mat or web of the multidirectional reinforcing material. Mat is available in various weights in the range of about 7/8 to 4 ounces per square foot. A specific type of multidirectional mat is a very lightweight mat of about 1/2 ounce per square foot or less that is known as a surfacing mat or veil and is used to provide a resin rich surface that enhances the surface finish of the article. Bidirectional reinforcing materials comprise woven fabrics having spaced fibers or rovings woven or knit at right angles to one another or at other selected angles such as 30.degree., 45.degree.and 60.degree.; this form of reinforcement material has some elements that provide axial reinforcement and others at an angle thereto to provide for transverse reinforcement. Glass fiber is the most widely used reinforcement material for pultruded plastic products, although other synthetic fibers including polyester fibers, nylon fibers, aramid fibers and carbon fibers are used but on a more limited basis.